The following is the explanation I got from Kayty
Ok so in dressage, and really general flat work, a horse that is using its body correctly, will 'swing' it's back as well as move its legs. When you see horses that trot around and look very flashy, with 'flicky' front legs - 99% of the time these horses are not using their backs and are just 'leg movers' - which look speccy but is not correct.
To have a horse working correctly, we want to create energy in the hind quarters, using our leg and seat, this energy will travel over the horse's back and up to the bridle, where we must keep a contact to 'complete the circuit' (think an electrical circuit, the energy will not flow if there is a gap in the circuit. It is the same with a horse. If there is a block or gap in the circuit, the energy will not flow through the whole horse, and the work is not 'correct').
As the energy we create in the hind legs, circulates up through the back and to the bridle, then cycling back which is where you get your collection, the back muscles will 'engage' and lift. In some horses, the 'lift' in the back is so obvious that you feel as though they've grown an inch. The back is then able to swing with the horse's movement, and carry the rider.
This is opposed to a horse that travels by bracing it's back and moving it's legs. The ride is not so comfortable and the energy circuit is not connected as the back is blocked.
We achieve the swing and 'throughness' by using half halts and other exercises to engage the hindquarters. You should feel the back 'release', and the horse's movement will immediately become freer flowing and much more pleasant to ride.
Some horses will naturally swing over the back, meaning they're very loose over the back and it makes the rider's job of creating that 'electrical circuit' much easier. Others, like most TBs, will travel with a naturally stiff, braced back, and as a rider we have to be skilled and tactful enough to encourage the horse to release its back and build those muscles so that eventually it becomes more comfortable to travel with a soft back versus a tense/stuck back.
So I was just wondering if I could get some pictures or videos or something to show the difference because I was never told about this before and would like to see. I'm not sure if you can see it in videos and such but Thanks if you can help me.